The present disclosure generally relates to additive compositions and processes for increasing the dry strength of a paper product.
Paper strength is generally characterized by its dry and wet strength, among other properties. The dry strength property can be measured as a function of its tensile strength as a dry paper sheet, which is typically conditioned under uniform humidity and room temperature conditions prior to testing. The wet strength property can be measured as the tensile strength exhibited by the paper product that has been fully dried and then rewetted with water prior to testing.
For many paper products, high dry strength coupled with low wet strength are desired. For example, many bath tissue grades require good water dispersibility such that wet strength should be avoided. Furthermore, paper broke is often sent back to the pulper for repulping during the papermaking process and high wet strength can cause paper re-pulping difficulty.
To increase dry strength, paper manufacturers often add dry strength additive compositions during the papermaking process. Many of these additives are cationic polymers, which have been found to improve papermaking retention/drainage efficiency, which can provide an increased machine speed. For example, cationic starches are often added during the papermaking process to increase dry strength without increasing wet strength. However, to provide further gains in dry strength, much research work has been carried out to replace the cationic starches with novel dry strength polymeric resins that exhibit improved performance. One of the most studied dry strength resins is polyelectrolyte complexes containing a cationic polymer and an anionic polymer.
By way of example, U.S. Pat. No. 3,332,834 describes a dry strength system comprised of an anionic polyacrylamide, alum, and a water-soluble non-thermosetting cationic resin having a molecular weight between about 1000 and about 30,000.
U.S. Pat. No. 4,824,523 generally describes a method for manufacturing paper that includes the step of adding a dry-strength retention agent system to paper stock prior to forming the paper. The system includes from about 1% to about 7% by weight of a cationic starch having a degree of substitution between about 0.01 to about 0.035; an anionic polymer characterized as a copolymer of acrylamide with acrylic acid or 2-acrylamide 2-alkylpropane sulfonic acid, wherein the anionic polymer has an average molecular weight greater than one million; and a non-starch cationic synthetic polymer.
European Pat. No. 0362770 generally discloses that mixture of cationic and anionic polymers are useful as a strengthening additive in papermaking processes The cationic and anionic mixture is characterized in that it comprises a water-soluble, linear, cationic polymer having a reduced specific viscosity (0.05 weight % in a 2 M NaCl solution at 30° C.) greater than 2 dl/g and a charge density of 0.2 to 4 meq/g, and a water-soluble, anionic polymer having a charge density of less than 5 meq/g that is reactable in the presence of water with the cationic polymer to form a polyelectrolyte complex.
U.S. Pat. No. 6,723,204 generally describes a dry strength resin that is an aqueous mixture of anionic dry strength resin and cationic starch or amphoteric starch having a net cationic charge, wherein the ratio of the dry strength resin to cationic or amphoteric starch is such that the aqueous mixture has a net cationic charge.
U.S. Pat. No. 6,616,807 generally describes a process for the production of paper, board and cardboard that includes the addition of cationic, anionic or amphoteric starch as a dry strength agent to paper stock and drainage of the paper stock in the presence of retention aids with sheet formation, wherein one of the following is used as a retention aid for starch: polymers containing vinylamine units, polyethyleneimines, crosslinked polyamidoamines, ethyleneimine-grafted and crosslinked polyamidoamines, polydiallyldimethylammonium chlorides, polymers containing N-vinylimidazoline units, polymers containing dialkylaminoalkyl acrylate or dialkylaminoalkyl methacrylate, polymers containing dialkylaminoalkylacrylamide units or dialkylaminoalkylmethacrylamide units, and polyallylamines.
U.S. Pat. No. 6,294,645 generally describes a dry strength system for paper comprising a cationic component and an anionic component, wherein the cationic component comprises a reaction product of an intralinker and a polyamidoamine. The polyamidoamine prior to reacting with the intralinker has a reduced specific viscosity of less than about 0.125 dl/g, wherein the intralinker to amine is in a ratio of 0.10:1 to about 0.40:1 on a molar basis and wherein the intralinker is selected from the group consisting of epihalohydrins and diepoxides.
While prior art formulations may be adequate for use as a dry strength additive, there is a continuing need for a product that provides improved dry strength to a paper product without increasing wet strength.